1 Introduction
A reduction of CO
2 emissions is necessary to meet the targets of the Paris agreement. Those targets should be mainly achieved by investing in wind and solar power, measures to increase the energy efficiency, reforestation, and reduction of deforestation [
1]. Today, renewable energy sources like hydropower and biofuels/charcoal mainly contribute to the production of electricity (about 18%) and heat (about 64%) [
2]. Austria already has a significant share of renewables in the electricity (about 76%) and district heat (about 44%) production [
3]. In contrast, the industrial and the transportation sectors mainly use fossil fuels to cover their energy demand. Especially, the industrial sector could lower its CO
2 emissions by using biomass-based processes. Since a direct utilization of biomass is always challenging in terms of purity and solids handling, it is a promising option to convert biomass in a first step into an intermediate feedstock like already applied fossil fuels, e.g., bio-oil, biomethane (generated via biogas upgrading), or BioSNG (generated via biomass gasification and downstream methanation, SNG = synthetic natural gas). The downstream process then needs no significant adjustment and existing assets can be used. However, biomass-based feedstock come with a significant price increase as suitable technologies are not yet mature, capacities are small, and fossil energy, electricity, natural gas, and oil are available at low price levels today. These aspects lead to a difficult economic situation for the implementation and commercialization of biomass-based concepts.
Among the biomass-based energy carriers, renewable methane is a promising candidate since its production according to current specifications is relatively simple and existing infrastructure of natural gas can be used. Today, biomass-based methane is mainly produced from organic feedstock based on cellulose or starch via anaerobic digestion and biomethane purification. In this process, CO
2 and trace components (e.g., H
2S, organics, and ammonia) are removed from biogas, i.e., the raw gas from fermentation mainly consists of CH
4 and CO
2, typically 50–55 and 45–50%, respectively. The separation of CO
2 is carried out by unit operations like absorption (e.g., amine scrubbing or pressurized water scrubbing), adsorption (PSA), or gas permeation through polymeric membranes [
4]. Although upgrading of biogas can be beneficial, the largest share of biogas plants are producing raw biogas for combined heat and power (CHP) operation [
5,
6]. Today, among the total worldwide installed capacity for biogas (plants with capacities between 1.5 and 10,000 m
3 h
−1 raw gas), 15% are for biomethane production [
7]. The largest players are Germany, Sweden, and the UK with approximately 2 GW installed capacity of biomethane [
7]. This distribution is mainly caused by local regulations. In Germany, as largest player in biogas production, since 2012, only plants with less than 75 kW electrical capacity (i.e., approximately 40 m
3 h
−1 raw biogas) based on manure get high funding rates (the so-called Güllebonus). Therefore, in the last years, predominantly small plants have been built. In such small plants, the production of biomethane is not beneficial due to high investment and disadvantageous economy of scale.
Those plants that are favored for biomethane production have higher capacities (on average about 900 m
3 h
−1 raw biogas, status 2016 [
7]) and most of them (82% in 2016, energy-based number) are based on energy crops, of which 84% are represented by corn [
5]. If an environmental-friendly and ethically correct production of biomethane on a significant industrial scale is targeted, alternative kinds of feedstock and technologies are required (for example, conversion of straw). The most promising option on the thermochemical conversion side is biomass gasification combined with a methanation to produce BioSNG.
Today, the generation of SNG via the thermochemical route by gasification is mainly carried out with coal as feedstock. The first commercial SNG production plant was the Great Plains Synfuels Plant in North Dakota, USA, employing lignite gasification [
8]. Furthermore, several coal-based SNG projects are ongoing in China or planned for the upcoming years with a total expected SNG capacity of 5 GW in 2020 and 20 GW in 2030 [
9]. However, these plants warrant a critical assessment regarding their CO
2 emissions and their general environmental impact [
10]. In contrast to coal-based SNG, the GoBiGas plant in Gothenburg, Sweden, is a dual fluidized bed (DFB) biomass steam gasification-based BioSNG plant which can generate CO
2-neutral SNG from biomass [
11]. However, due to hard market conditions, the plant was mothballed at the beginning of 2018 [
12]. In addition, a BioSNG pilot plant employing fluidized bed methanation was extensively investigated at the site of the commercial DFB CHP plant in Güssing, Austria [
13,
14].
Biomethane from biogas and BioSNG production based on the thermochemical route are not to be considered as competitive technologies. Besides the different feedstock range for the processes, their different scalability make them both viable options for production of a renewable natural gas (NG) substitute. For both technologies, the injection regulations are defined in the standard EN 16723-1:2016.
In addition to an energetic utilization of biomethane/BioSNG, the large potential of biomethane and BioSNG in the chemical sector (e.g., H
2 production, methanol synthesis, and ammonia synthesis) can be pointed out. Hence, several sectors, transportation, heating, electricity generation, and chemical industry can benefit from a renewable production of methane as a key intermediate [
15].
For the use of renewable methane in the transport sector, the entire production chain needs to fulfill the requirements of the EU Renewable Energy Directive [
16] and its additions. The key parameters are the total greenhouse gas emissions, which need to be at least 60% lower than the reference case. This is valid since January 1, 2018, for plants that are commissioned after January 1, 2017 [
16]. Since the emissions of all greenhouse gases have to be taken into account, a careful analysis of the overall process and product lifecycle is required. In this paper, a first evaluation of the expected GHG emissions will be made as an addition to the techno-economic assessment of the respective processes.
4 Conclusion and outlook
The results show that the natural gas substitute selling price of the BioSNG process is significantly lower at larger plant capacities of up to 50 MW natural gas substitute output. At 50 MW, the gasification-based route starts to get competitive with the biomethane processes. However, about 50 MW biomethane power is the output of the already largest biomethane production facility in Germany [
34]. Therefore, at the moment, 50 MW biomethane output seems to mark the upper capacity for anaerobic digestion plants when feedstock availability is especially considered. Biomethane plants of this size usually do not produce according to the RED because of the used feedstock (maize silage). Consequently, from this scale upwards, the gasification-based BioSNG concepts with wood as feedstock are getting economically attractive due to their scalability and feedstock availability with a lower environmental impact. Smaller units may be attractive if cheaper feedstock can be used. In general, feedstock availability plays a vital role for both routes. Moreover, the feedstock is a major factor for the greenhouse gas emission for both technologies. However, both routes can employ different feedstock and can therefore coexist as both will be needed to replace fossil natural gas in the future. Nevertheless, biomethane plants are a commercially available and employed technology, whereas BioSNG plants are not in commercial operation so far.
To increase the economics, co-feeding of cheap low-quality feedstock is an option which needs to be extensively investigated. In addition, the sensitivity analysis showed that the annual operating time of the plants has significant influence on the economic feasibility. Therefore, measures should be implemented to achieve sufficient annual operating times as well as high process reliability.
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